PLANETPLANET

This is chapter 3 in the Solar System’s story. We’re chugging along, growing bigger and bigger things…

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Planetesimal accretion

After mountain-sized (~100 km-scale) planetesimals form from concentrations of drifting pebbles, they continue to grow in two ways. The simplest growth route for planetesimals is simply to crash into other planetesimals; this is called planetesimal accretion. But not all collisions led to net growth, because planetesimals are a lot weaker than you might imagine. When two crash together they often break apart and create a lot of debris. After high-speed collisions, the resulting merger can even be smaller than the original objects. It’s like throwing two giant Lego castles at each other: even though their pieces could add up to something bigger, bashing them together will not get you any closer.

But all is not lost. The biggest planetesimals keep growing by colliding with smaller ones, even as those smaller ones are ground into dust. Life just isn’t fair if you’re a puny planetesimal — you’re smashed to pieces that are just used to make the biggest ones bigger. It’s like taking pieces off of the smaller Lego castles to beef up the biggest one (Hogwarts, I presume).

Here are snapshots in a computer simulation of a sea of planetesimals growing by planetesimal accretion:

Pebble accretion

There is another process that is even less fair to the little guy. It is called pebble accretion. As with planetesimal accretion, the biggest planetesimals grow faster than the rest. This time, their accelerated growth is a result of their larger gravity, which allows them to grab onto some of the pebbles that continue to drift by through the gas disk. This process works for the biggest planetesimals and gets even more efficient for planets (until it stops, as we shall see).

This animation shows how fast pebble accretion can be — a small planetesimal is just gravitationally deflected by a really big planetesimal (at the center) but a pebble spirals down and keeps the growth going.

With enough pebbles drifting by, a big planetesimal grows really fast. And we know there are a lot of pebbles in planet-forming disks because we can directly observe them with the ALMA sub-millimeter telescope.

Planetary Embryos

As planetesimals continue to grow — by accreting pebbles or planetesimals — they eventually reach a size scale of “planetary embryos” (sometimes called ‘protoplanets‘). In the rocky planet region, planetary embryos were probably about the size of the Moon or Mars — about 1-10% the mass of Earth.

But farther from the Sun, we think that planetary embryos were much larger, probably five to ten or even twenty times the mass of Earth. (As we shall see, these are the cores of the giant planets). Why were embryos so much bigger farther from the Sun? Probably because pebble accretion was likely much more efficient. The current thinking is that past the snow line, pebbles were centimeter-sized agglomerations of dust held together by ice. But in the hotter regions close to the Sun, the ice vaporized and the remaining large dust grains were much smaller, only millimeters in size. This size difference of pebbles matters: accretion is exponentially faster for larger pebbles. In the time it took for the giant planets’ cores to accumulate five to ten Earth masses of ice-rich pebbles, the rocky planetary embryos could only grow to roughly the size of the Moon or perhaps Mars.

It looks something like this (from left to right):

Turning off pebble accretion

Like all good things, pebble accretion eventually comes to an end. As a planetary embryo grows, it becomes more and more efficient at grabbing onto pebbles that are continually drifting by, until it is eating a large fraction of them. But at the same time, the growing embryo creates a density wave in the disk outside its orbit. This density wave acts to trap drifting pebbles before they can reach it. These yummy pebbles are dangling just out of reach, like the proverbial carrot. When this happens, the growing planet is said to have reached its “isolation mass”, because it has grown so massive that it has created a barrier between itself and the pebbles drifting by.

At this point, the planetary embryo stops growing (at least from pebble accretion). But so do all other planetary embryos and planetesimals closer to the Sun! This means that, once Jupiter’s core grew massive enough it would have stopped pebbles from entering the inner Solar System. It’s kind of like one kid having a tantrum and ruining the game for everyone.

Of course, once planetary embryos get big enough to cut off the flux of pebbles, they are subject to a whole different effect: orbital migration (coming in chapter 4).

Wrap-up

The TL;DR version of this post: planetesimals grow by colliding with other planetesimals (planetesimal accretion) or by grabbing onto drifting pebbles (pebble accretion). Planetary embryos were Moon- to Mars-sized in the rocky planet zone and many Earth masses in the giant planet region.

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Additional resources

• The Solar System’s story
• Who’s scruffy-looking? (maybe not terribly relevant but one of the great movie lines ever)